Engineering
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223553 / 9160625310
2018-05-24·BA00·III·en·04
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Transmitter power supply
N-132...IL
6.3
Engineering of the Power Dissipation in Cabinets
When installing devices in cabinets, the free flow of air is limited and the temperature
rises. It is important to optimize power dissipation and the heat generated in the cabinet
in order to minimize increases in temperature.
a) Natural convection in closed cabinets
• Application: If the power dissipation is low and the system is installed in a dusty or
harsh environment.
• Calculation of the maximum permissible power dissipation:
The calculated value P
max
must be less than the sum of average power dissipation
values (70 % of the max. power dissipation) of the installed devices: P
max
< Ʃ P
70%
b) Natural convection in open cabinets
• Function: The heat is displaced to an area between the devices by cool air streams.
• Prerequisites:
- Air inlet and outlet openings at the lower and upper ends of the cabinet
- The airflow path must be free of obstructions
• Result: Depending on the version, a value that is double the permissible power
dissipation can be reached as shown under a).
c) Forced ventilation with heat exchanger in closed cabinets
• Application: If either the environment or the high power dissipation does not allow for
any natural convection.
• Function: A heat exchanger with fan suctions air into the cabinet and guides it to the
heat exchanger plates, which are cooled by a second fan that expels ambient air.
• Result: Depending on the version, a value that is 5 or 6 times the permissible power
dissipation can be reached as shown under a).
P
max
=
D
t * S * K
P
max
[W]
Max. permissible power dissipation in the cabinet
D
t [°C]
Max. permissible temperature rise
S [m
2
]
An unobstructed, heat-emitting surface of the cabinet
K [(W/m
2*
°C)] Thermal conductivity coefficient (painted steel: K = 5.5)
